One of the most important characteristics of machine operation used in the analysis of vibration data is the rotational speed of the machine. Machine speed is often acquired from tachometer measurements. Accurate tachometer pulses are essential for applications such as order tracking, synchronous time averaging, single channel phase, Bode plots, and general turning speed calculations. Ideally, tachometer pulses are generated when illumination from a tachometer sensor passes over a section of reflective tape placed on a rotating portion of the machine, such as a rotating shaft. However, the shafts of most machines in industry do not have reflective tape. If they have reflective tape, often times the tape is covered with grime or has rubbed off or is otherwise damaged to some extent. Because a machine typically cannot be stopped in order to place reflective tape on the shaft, an analyst will collect tachometer data in hopes of getting a trigger from a keyway, a scratch on the shaft, or remnants of old reflective tape. The resulting tachometer waveform is typically noisy and small in amplitude and the tachometer pulses are not very distinctive.
An ideal tachometer waveform consists of a series of distinct pulses, wherein each pulse indicates a single revolution of the shaft. To generate a tachometer pulse sufficient for the applications mentioned above, a threshold is set such that a rising (or falling) edge of the tachometer signal triggers a pulse when it passes the threshold. This threshold is manually entered by an analyst. In an ideal situation, where reflective tape is present resulting in a strong tachometer signal, the chosen threshold value will generally be acceptable for all measurements. However, when reflective tape is not present, the level of the tachometer signal varies from one machine to the next and the necessary threshold will be different for each measurement.
What is needed, therefore, is an automatic method for calculating a proper threshold for a given tachometer signal in order to produce the desired tachometer pulses needed for machine vibration analysis.